The invention declared herein is an electric component comprising of distributed RC-structures and having a trimmable resistive-capacitance impedance, i.e. a trimmable complex impedance device or a stub.
Resistors, capacitors and inductances which are trimmable electric components are known in the prior art. To realize a complex impedance, parallel or series connections of the above mentioned components or distributed RC-structures have been used. However, a trimmable complex impedance has not yet been presented and trimming, where necessary, is carried out by successive trimmings of a resistor and a capacitor connected in parallel or in series, the trimmings taking place at different points of time.
Since modern electronic circuits are often realized as integrated film circuits such as thick film and thin film circuits, where only one component can be trimmed at a time, only permanently and only in one direction, the trimming of a complex impedance or, in general, the trimming of an electric circuit by changing the complex impedance thereof, is very difficult. That is the reason for aiming, already during the design stage, at solutions where a desired characteristic feature of the circuit could be adjusted by trimming only the resistor, that is to say, the resistive impedance.
The distributed RC-structure which is used in the present invention has been presented, for example, in an article by W. M. Kaufman: "Theory of a Monolithic Null Device and Some Novel Circuits", pp. 1540-1545, Proc. IRE, September 1960, and in the British Pat. No. 1,149,764.
The British Patent deals with two notch circuits. In this context, however, the trimming of a complex impedance cannot be learned. The first notch filter consists of a coaxial transmission line, to which a grounding resistor has been connected. A good conductor in the middle of the coaxial transmission line is encircled by an insulation layer which in turn is encircled by a resistive outer layer. The notch is trimmed by shortening the transmission line gradually until the desired notch-frequency is achieved.
It is pointed out in the same patent that a planar structure can be used instead of the coaxial transmission line. It must be noticed, however, that the notch described above cannot be manufactured at an exact frequency because of the statistical divergence of the RC-product of the RC-structure formed by the transmission line. Besides, the trimming must take place before the attaching of the connecting wires.
The above mentioned British Patent also discloses a trimmerlike, adjustable notch filter. Therein the values of the two components, the resistive-capacitive and the resistive ones, can be adjusted reversibly and in parallel by using a slide. However, the manufacturing of such a circuit is difficult because the demanded ratio 17.8 between the resistivities of the components cannot be realized in a satisfactory manner. Thus, the circuit does not function in the desired way. It would also be impossible to use such a circuit in integrated circuits.
Various kinds of electric circuits are presented in the U.S. Patent 3,109,983. The production of these circuits has not been practical because the manufacture divergence of their distributed RC-structures cannot be eliminated by trimming only the resistor or the capacitor. The trimmable impedance device of the present invention allows for the practical manufacturing of these circuits, as well, which fact has been confirmed by various experiments.
Clearly, there is a need for a component, in which both the resistive and the capacitive parts of the impedance are changed simultaneously in a fully predictable way and by a single trimming operation. The purpose of the present invention is to realize a component with a trimmable electric impedance, i.e. a stub. This is achieved by the characteristic features of the invention.
Before this invention it has not been possible to produce a component, the impedance of which could be trimmed in an exact and predictable way. The advantages of the inventive component are, above all, the accuracy of the trimming and the possibility to calculate the characteristics of the circuit exactly and in advance. These advantages are remarkable particularly when the component is part of an integrated circuit.